Press-on, twist-off bottle cap

ABSTRACT

The closure cap herein described has a generally cylindrical skirt with a circular lowermost edge. Corrugations in the form of axial ridges and flutes terminate downwardly in a plane spaced from the edge. The crests of the ridges and the cylindrical lip are elements of a common cylinder. When the cap skirt is uniformly constricted about threads of a bottle finish, material along the lines of the flutes forms female thread elements, securing the cap to the bottle. The ridges, the thread elements and the lip constitute a knurling grid which affords a comfortable, sure finger grip for twisting the cap off the bottle, with the smooth lip surface shielding the edge.

United States Patent Moller 1 Mar. 14, 1972 i541 PRESS-0N, TWIST-OFFBOTTLE CAP [72] Inventor: Jens L. Moller, Westmont, Ill.

[73] Assignee: Continental Can Company, Inc., New

York,N.Y.'

[51] Int. Cl. ..B65d 41/20, 865d 4l/l0, 865d 41/04 [58] Field of Search..215/40, 43, 42, 39

[56] References Cited UNITED STATES PATENTS 2/1967 Osborne et a]..215/42 X 9/1969 Dorn et a1 ..2l5/40 X Primary Examiner-George T. HallAttorneyGeorge E. Szekely, Joseph E. Kerwin and William A. Dittmann [57]ABSTRACT The closure cap herein described has a generally cylindricalskirt with a circular lowermost edge. Corrugations in the form of axialridges and flutes terminate downwardly in a plane spaced from the edge.The crests of the ridges and the cylindrical lip are elements of acommon cylinder. When the cap skirt is uniformly constricted aboutthreads of a bottle finish, material along the lines of the flutes formsfemale thread elements, securing the cap to the bottle. The ridges, thethread elements and the lip constitute a knurling grid which affords acomfortable, sure finger grip for twisting the cap off the bottle, withthe smooth lip surface shielding the edge.

8 Claims, 12 Drawing Figures PATENTEDMAR 14 B72 SHEET 1 [1F 2 INVENTORJENS L MOLLER PATENTEDMAR 14 I972 SHEET 2 [IF 2 INVENTOR wilt]?PRESS-ON, TWIST-OF F BOTTLE CAP BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to a bottle cap of the press-on,twistoff type. More particularly, the cap of this invention is of thetype adapted for formation of thread elements in the skirt uponapplication of the cap to the threaded finish of a bottle so as tosecure the cap thereto.

2. The Prior Art Crown caps have been generally used for beverage bottleclosures. A crown cap has a corrugated skirt which is normally crimpedunder a collar or head on the bottle neck. It has been proposed to applya crown cap to a finish having external threads or thread formations toform a twist-off closure. However, the short, flared skirt of a typicalcrown cap does not afford sufficient thread engagement upon crimping toassure a secure closure with dependable twist characteristics. The sharpcorrugation ridges and the undulated, projecting sharp edge do notafford a satisfactory grip for twisting the cap. The cap is often hardto turn and the user may cut his fingers on the edge. Reclosure isusually difficult and impractical.

It has also been proposed to provide a cap of the crown type with aprotected edge. The lip of the cap may be curled, bent inwardly orotherwise formed to shield the edge when the cap is on the bottle. Suchcaps usually involve additional operations, additional material, orboth, as compared with crown caps heretofore in more general use. Eventhe relatively small cost increases are a considerable disadvantage in ahighly competitive market. Furthermore, there remains the difficulty offorming adequate threads in the corrugated skirt typical of crown caps.

Caps suitable for roll-on thread formation have been marketed. For thispurpose, the cap is usually made of aluminum and is provided with acylindrical skirt which slips over the bottle threads when the cap ispushed on. While the cap is held in sealing position against the bottlelip, threads are rolled in the skirt, the threaded finish constituting adie.

The material costs for the aluminum roll-on cap are generally ratherhigh. It is not ordinarily feasible to use commercial tin plate or thelike, because low cost varieties of such material do not havesatisfactory thread-rolling characteristics. The thread rollingoperation involves extra equipment and is generally slow, as compared tothe usual crown crimping operation. Furthermore, thread rolling requiresthat most of the skirt be free of corrugations or other suchreenforcement, so that the skirt tends to warp due to irregularstresses, fatigue and the like. Hence, the twist-torque and reclosurecharacteristics of the roll-on cap are often unsatisfactory. Reinforcingthe skirt by means of beads, curls, pockets or similar formationsinvolves special tooling, extra operations, or both, still furtherincreasing the cost of manufacture.

SUMMARY OF THE INVENTION The cap according to this invention is designedto overcome the several above-noted disadvantages of previously proposedcaps, affording a dependable press-on, twist-off cap for beveragebottles and the like at a cost which compares favorably with that ofregular crown caps.

The cap has a generally cylindrical skirt depending from a top panel.The skirt terminates downwardly in a circular edge. The skirt is formedfrom single-thickness sheet material and includes a corrugated portionwhose length is about equal to the overall length of the threads on thefinish. The circumferential series of corrugations define straightflutes and ridges, the crest elements of which constitute elements of acommon cylinder. The ridges extend downward from a plane just below thetop panel to a plane spaced from the edge. The lower skirt or lip,extending from the lower termini of the ridges to the edge, is acylinder of substantially the same diameter as that of the ridge crestelements.

The root elements of the flutes lie on a cylinder sized for closetelescopic fit over the external threads on the finish to which the capis to be applied. The depth of the flutes is greater than the thicknessof the skirt material and about equal to the depth of the finishthreads. When the skirt is uniformly constricted about the threadformations, female thread elements are fonned in the skirt by bending ofmaterial along the lines of the flutes, but otherwise the skirtmaintains its generally cylindrical configuration. The flute depth beingnot less than the effective thread depth, the constricted lip will clearthe crests of the finish threads when the cap is twisted off.

The cap according to this invention can be readily formed from sheetmaterial in a single stroke of a double-acting punch press, using asimple blank-and-form die set. The cap configuration lends itself to useof light-gauge tin-free steel, minimizing stress cracking and spoliationof coating and decoration. The corrugations are more numerous than isusually feasible on crown caps. The closely spaced ridges provide a highdegree of radial stiffness, minimizing buckling upon constriction, sothat the skirt thread elements can be well-conformed to the finishthread formations, providing secure retention and minimal tendency towarping of the skirt. A greater lineage of thread engagement with givenskirt length is obtainable than with the corrugations usually providedin standard crown caps.

After constriction, the outside diameter of the skirt across the majordiameter of the thread elements is about the same as the diameter of theridge crests and the subjacent smooth lip. The resultant surfaceconstitutes a knurl grid which affords a sure and comfortablefinger-grip for twisting the cap off and on the bottle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of abottle cap according to this invention;

FIG. 2 is an enlarged, fragmentary elevational view similarly to FIG. 1,a portion of the cap being shown in vertical section taken through oneof the corrugation ridges;

FIG. 3 is a fragmentary vertical sectional view similar to the sectionalportion of FIG. 2, but taken through the root of one of the corrugationflutes;

FIG. 4 is a fragmentary horizontal sectional view taken on line 4-4 ofFIG. 2, showing the corrugation contour;

FIG. 5 is a view similar to FIG. 4, showing a variant of the corrugationcontour;

FIGS. 6 and 7 are views similar to the sectional views of FIGS. 2 and 3respectively, but showing the cap applied to a bottle, preparatory toconstriction of the skirt;

FIG. 8 is an elevational view of the cap applied to a bottle, afterconstriction of the skirt about the finish;

FIG. 9 is an enlarged fragmentary view similar to FIG. 8, partially insection, showing details of the skirt thread elements;

FIGS. l0, l1 and 12 are views similar to FIGS. 2, 3 and 9 respectively,showing a modified form of the cap.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, cap 10 isproportioned for application to thread formations on the finish of abottle such as is used for carbonated beverages. Skirt 11 is generallycylindrical, terminating downwardly in a circular edge 12. The skirt 11is sized to telescope closely over the finish and to be secured theretoby substantially uniform constriction of the skirt.

As seen in FIG. 2, cap 10 has a domed top panel 13, which is connectedto skirt 11 by a radiused portion 14, which defines a seal recess, assubsequently seen. Skirt 11 is formed with a circumferential series ofcorrugations l5 defining outwardly projecting axial ridges l6 and flutes17, extending from the bottom of portion 14 to lowermost termini 18 in ahorizontal plane spaced from circular edge 12. The surface of lip 19,ex-

on a cylinder of substantially the same diameter as the cylinder onwhich lie the crest elements of ridges 16. As best seen in FIG. 3, thedepth of flutes 17 is somewhat greater than the thickness of thematerial of skirt 1 1, the depth being about that of thread elements tobe formed in the material along the lines of the flutes.

Referring to FIG. 4, the preferred corrugation pattern is characterizedby ridges l6 somewhat narrower than flute width W. The cap may be madeof 55 No. steel sheet. A cap of that material for a standard beveragebottle opening, preferably has about 40 corrugations. This corrugationpattern provides the radial stiffness and circumferentialcontractibility requisite to retention of the generally cylindricalconfiguration upon application of radially inward pressure uniformlyaround and along the skirt. The material indicated is substantially lesscostly than the 75-90No. tin plate in general use for conventional crowncaps intended for the same service conditions as the cap here shown. Theroot material is conformed to the inner circle, providing a substantiallineage of material lying thereon. The material defining the flutes 17being thus preformed, fairly high temper characteristic ofdouble-reduced, light gauge material is feasible, requiring onlymoderate constrictive force to form thread elements therein. Less severedeformation is involved in forming thread elements along the lines ofthe flutes than is the case with typical crown cap corrugations.

FIG. 5 shows another corrugation pattern for the same size cap as thatof FIG. 4. In the pattern of FIG. 5 the corrugations are generallysinusoidal, the flutes 217 having substantially the same radii as theridges 216. This corrugation pattern provides generous radii at crestsand roots which minimize malformation and tearing in the formingoperation.

FIGS. 6 and 7 show cap 10 in position upon a bottle 20, preparatory toconstriction of skirt 1 1 about the thread formations 21 on finish 23. Agasket or liner 24 extends along panel 13 and radius 14, so as to sealon top and around the side of lip 25, when cap 10 is pressed down.Flutes 17 are substantially coextensive with the axial extent of threads21, the material of the skirt along the lines of the flutes closelyopposing the crests of the threads. The ridges l6 and lip 19 areradially spaced from threads 21, the clearance corresponding to thedepth of the flutes.

Cap 10 can be applied in a crowning machine of the type generally usedfor applying crown caps. Suitable tooling is indicated fragmentarily inphantom lines, comprising sealing punch 40, sleeve punch 50, squeeze die60 and back-up ring 70. Die 60 is made of an elastomeric material,rubber for example. While cap 10 is held in the sealing position shown,by means of punch 40, punch 50 descends, compressing die 60 against ring70. Die 60 deforms and exerts uniform inward radial pressure around andalong skirt 11, constricting it about finish 23.

FIGS. 8 and 9 show the cap 10 after constriction of skirt 11, withfemale thread elements 22 formed therein upon constriction. As hereshown, threads 21 comprise half-turn elements of a compound thread. Thepitch is somewhat greater than the thread width at the pitch circle.This is a preferred form of threading, providing ample threadengagement, yet with wide roots to accommodate thread elements 27 formedby radially inward displacement of material along the lines of flutes 17between threads 21. The coarse thread pitch will establish rather highlocalized bending stresses in the skirt material bearing along thecrests of threads 21. This assures close conformation of the skirtmaterial along the lines of flutes 17, across and about the threads 21,thereby creating a number of female thread elements 22 engaging elementsof finish threads 21. The total pitch-line lineage of engagement isabout equal to WxNxT, wherein W is the flute width (FIGS. 4, 5), N thenumber of flutes and T the number of thread turns of the finish threads.This structure provides excellent retention characteristics under highthrust, even with a large thread helix angle. Compounding the threads21, as shown, enhances ease of removal and replacement of the cap. Ashort twist frees the cap for removal and develops a tight seal onreclosure, at moderate torque.

When uniform constrictive forces are applied, the material of the skirtalong the lines of flutes 17 bears on the crests of the threads 21. Thematerial bends over and around the threads due to the high localizedbending stresses developed. The material defining the ridges 16 does notbear on the threads. Hence, substantial bending stresses do not developin the ridge material. Furthermore, the ridges 16 are sufficiently stiffto obviate buckling under shear stresses imposed by bending of thematerial along the flutes. The ridges 16 remain straight, whileproviding the accordion effect to evenly accommodate reduction of theskirt diameter. This accordion ef fect obviates irregular buckling orwrinkling. Upon completion of the constrictive deformation, the diameteracross the crest elements of ridges 16 is less than the originaldiameter by an amount about equal to the double depth of the threads 21.Also, the outside diameter of the skirt across the major diameter of thefemale thread elements 22 is about the same as the diameter across thecrests of the ridges 16.

The cylindrical contour of lip 19 is substantially unaffected byconstriction stresses, the lip being restrained against scalloping orcurling by the stiffening effect of the ridges 16. Upon constriction,lip 19 has an inside diameter slightly greater than the major diameterof thread formations 21. Then, upon unscrewing cap 10, lip 19 clearsthread formations 21.

The length of flutes 17 is substantially the same as the total axialthread length of the threads 21, thus assuring ample thread engagement.The depth of the flutes is about equal to the depth of the threads 21,this dimensioning being conducive to fonning well-defined threadelements 22, closely conformed about the threads. Because of thestiffening effect of the ridges 16, the constricted cap has goodshape-retention characteristics, conducive to true-running threadengagement, for low-torque twist-off and good reclosure characteristics.The general stiffness of the skirt is such as will afford goodresistance to warping, which might otherwise result from irregularitiesor locked-in stresses developed either in originally forming the cap orin forming the closure.

As previously observed, lip 19 is cylindrical. The crests of ridges 16and thread elements 22 are of the same diameter. being elements of acommon cylinder, and are rounded. The generally cylindrical grid thusformed constitutes a fine knurl with smoothly contoured outermostsurfaces, affording a comfortable, sure finger grip. The skirt can begripped firmly without cutting or abrading ones fingers. With moderategrip pressure required there is little risk of a finger being cut bycurling over the edge. Easy twisting of the cap is facilitated by thewell-formed threads in the skirt and minimal tendency to ovality,warping, or cocking, which assures a low ratio of torque to sealingpressure, yet with relatively coarse threading, as preferred for quickrelease and reclosure. These easyopening features are particularlyadvantageous in soft-drink bottles which are commonly opened bychildren.

The configuration of FIG. 4 is particularly conducive to good threadformation and retention characteristics. Root elements 217 are initiallyinwardly concave. Less severe deformation is involved in conforming thematerial upon and about the threads 21. The necessary constriction ofthe skirt can be generally achieved with somewhat less radial pressurethan will be the case of the inwardly convex root configurations of thecorrugations shown in FIG. 5. However, the choice as to number and formof corrugations requires giving due consideration to the properties ofthe cap material, cap size, thread form, bottle finish quality andrelated factors.

MODIFICATION FIGS. 10 and 11 illustrate a tamper-proof form of the cap,parts corresponding to those in FIGS. 1-9 being given the same referencenumbers with the addition of 100. Cap is similar to cap 10, except forgreater skirt length beyond the corrugations, corrugation length beingthe same in both caps,

for the same threading. Skirt 111 depends from panel 113 along radiusportion 114, with ridges 116 and flutes 117 having termini 118 in aplane spaced from the edge 112. However, there is a longer cylindricalportion 119 between the plane of corrugation termini 118 and edge 112than the corresponding marginal portion or lip 19 of the regular cap.

Portion 119 is lanced intermediate the corrugation termini 118 and edge112, so as to cut a circumferential series of narrow slots 130. Then aseries of short, frangible bridges 131 connect the upper and lower bandsof marginal portion 119. The orientation and spacing of bridges 131 areso selected that each bridge is aligned with a ridge 116. Except forminor adaptation of the tools to accommodate the longer skirt and forthe lancing operation, cap 110 can be made in the same manner as cap 10.

Aluminum is a preferred material for the cap 110. Aluminum is usuallymore satisfactory and economical than steel for caps of the tamper-prooftype. Aluminum ge'nerally provides the most desirable combination ofductility for forming and requisite frangibility of the bridges.

Referring to FIG. 12, cap 110 is applied to the finish 120 in the samegeneral manner as above described with respect to the regular cap ofFIG. 1. Lip 132 is crimped under shoulder 133, below threads 121. Thiscrimping operation can be performed simultaneously with the generalconstriction of the skirt, by means of a compressible die similar to die60 (FIG. 6). Lip 132 being remote from ridges 116, and aluminum beinghighly ductile, a generally uniform radial constrictive pressuresufficient to conform skirt 116 to the thread formations 121 willusually suffice to crimp the lip 132 against shoulder 133.Alternatively, lip 132 can be crimped, rolled or spun under with anauxiliary die or tool. Collar 135 prevents excessive constriction ofskirt portion 134 and bridges 131. Cap 10 can also be applied tothisstyle of finish, if desired.

After completion of the closure as described, the tamperproofing meansof cap 110 operates in the same manner as with other caps having similarfrangible bridge structure. The engagement of inturned lip 132 undershoulder 133 inhibits rise of the cap upon initial application oftwist-off torque. Thereafter the twist torque and camming action of thethreads impose sufficient stress on bridges 131 to fracture them. Thelip portion 132 then drops down, visually signalizing attempted opening,even though the body of the cap be not actually twisted off. After thebridges are broken, the body of the cap can be twisted off to open thebottle. The body of cap 110 is substantially the same as cap 10, as toform, thread formation, grip, easy twist-off and reclosure. The insidediameter of the constricted skirt portion 134 and the bridge circle areslightly larger than the major diameter of threads 121. Hence, the bodylip and remnants of the bridges will clear the threads when the body ofthe cap is unscrewed.

Referring again to FIGS. 2, 3, 9 and 10, it will be seen that the capshell of either type can be readily formed from sheet material in apunch press with simple blank-and-form tooling. The equipment and set-upused can be substantially that used for manufacturing conventional crowncaps, with comparable production rates and overall economy. Dies for theregular and tamper-proof types may be of similar design, differingessentially only in blank size and depth.

The regular cap of FIG. 1 has a skirt 11 of about the same length asthat of a conventional crown cap for a similar bottle. Hence, the blanksize for the plain cap of the invention is about the same as the blankfor the crown cap; correspondingly, the same quantity of caps in eachcase can be made from about the same square footage of material. Sincethe regular cap here described can be produced from low-cost tin plateor tin-free steel of lighter gauge than has been found feasible withcrown caps, for reasons above noted, the total material and cost thereoffor the cap of this invention is generally less than for conventionalcrown caps.

The tamper-proof cap of FIGS. l012 has the same length and form abovebridges 131 as the complete regular cap, for the same bottle size andfinish. Only so much additional material is required as corresponds tothe width of the tamper proofing band 132. Either regular ortamper-proof type may be made shorter than the nearest equivalentpractical roll-on type of cap.

The caps according to this invention are adaptable to relatively finethreads, because of their suitability to thread formation by means of acompressible die. in contrast, a cap designed for roll-on threadformation is generally suitable for application only to fairly wide,coarse-pitch threads. The localized high tool pressure involved withthread rolling necessitates liberal tool bearing spun to obviate tearingof the cap skirt and to minimize tool or bottle breakage consequent uponjamming of the solid rolling tool between threads or excessive sidepressure on the tool. correspondingly, a roll-on cap generally requiresa substantially longer skirt than that of an equivalent cap of thisinvention for comparable thread engagement. Thus, the caps heredescribed afford greater economy of material as compared with roll-ontypes, with respect to both blank size and gauge. Lighter gauge isfeasible due to lesser risk of tearing or warping when applying uniformoverall pressure, as compared with the highly concentrated pressure ofthe thread-rolling operation.

It will be seen from the foregoing that the closure cap according tothis invention affords improved performance and economy, as compared toprior caps employed in similar applications. The caps above describedare designed for application to the threaded bottle finish shown by wayof example. It will be understood that the cap of this invention isadaptable to thread forms other than that here shown, or to threadequivalents such as cam lugs, or the like.

What is claimed is:

1. A closure cap for application to a bottle or the like, said capcomprising: a top panel with a generally cylindrical skirt dependingtherefrom and terminating downwardly in a circular lowermost edge, atleast that portion of said skirt which extends from a first plane belowsaid panel to said edge being formed of sheet material of substantiallyuniform single thickness, said skirt portion being corrugated so as todefine a circumferential series of straight outward ridges and straightflutes therebetween extending from said first plane to a second planespaced from said edge, the crests of said ridges and the outer surfaceof said skirt from said second plane to said edge lying on a commonsubstantially cylindrical surface.

2. The invention as defined in claim 1, said corrugated skirt portionbeing sized for constriction about external thread formations on thefinish of a bottle so as to deform inward portions of said materialalong the lines of said flutes over and about portions of the threadformations and thereupon to form female thread elements 'in said skirtconforming to the external thread formations.

3. The invention as defined in claim 2, the depth of said flutes beingapproximately the same as the depth of the external thread formations,whereby, subsequent to said constriction, the outside diameter of saidskirt across the major diameter of said thread elements will besubstantially the same as the diameter of said cylinder.

4. The invention as defined in claim 3, the depth of said flutes beinggreater than the thickness of said material, whereby upon generallyuniform said constriction of the entire said skirt below said firstplane, the inside diameter of said skirt between said second plane andsaid edge will be not less than the major diameter of the externalthread formations.

5. The invention as defined in claim 1, said skirt being weakened arounda circle in a plane between said second plane and said edge.

6. The invention as defined in claim 5, said cap being adapted forapplication to a bottle having a downwardly facing external shoulderbelow the thread formations on the finish, said skirt being of suchlength that a lip portion thereof below said circle of weakening willextend below the shoulder when said cap is applied to the bottle,whereby said lip portion can be formed into engagement with the shoulderso as to form a tamper-proof closure for the bottle.

7. A twistably removable closure cap for engaging external threadformations on the finish of a bottle or the like, said cap comprising: atop panel and a generally cylindrical skirt depending therefrom andterminating downwardly in an edge, at least that portion of said skirtwhich extends from a first plane below said panel to said edge beingformed of sheet material of substantially uniform single thickness, saidskirt being corrugated so as to define straight outward ridges extendingfrom said first plane to a second plane spaced from said edge, therebeing flutes between said ridges, portions of said skirt along the linesof said flutes being conformed to elements of the external threadformations so as to define female thread elements between said ridges,the crests of said ridges and a circumferential band of said skirtimmediately downward of said second plane being elements of a hollowcylinder having an inside diameter not substantially less than the majordiameter of the external thread formations, the other portions of saidskirt below said first plane lying substantially within the outsidediameter of said cylinder.

8. The invention as defined in claim 7, said ridge crests and the crestsof said female thread elements being rounded, the outside diameter ofsaid skirt across the major diameter of said female thread elementsbeing substantially equal to the outside diameter of said cylinder, saidridges, said thread elements and said band thereby defining a gridaround said skirt adapted to shield said edge and to afford a sure andcomfortable finger grip for twisting said cap off and on said finish.

1. A closure cap for application to a bottle or the like, said capcomprising: a top panel with a generally cylindrical skirt dependingtherefrom and terminating downwardly in a circular lowErmost edge, atleast that portion of said skirt which extends from a first plane belowsaid panel to said edge being formed of sheet material of substantiallyuniform single thickness, said skirt portion being corrugated so as todefine a circumferential series of straight outward ridges and straightflutes therebetween extending from said first plane to a second planespaced from said edge, the crests of said ridges and the outer surfaceof said skirt from said second plane to said edge lying on a commonsubstantially cylindrical surface.
 2. The invention as defined in claim1, said corrugated skirt portion being sized for constriction aboutexternal thread formations on the finish of a bottle so as to deforminward portions of said material along the lines of said flutes over andabout portions of the thread formations and thereupon to form femalethread elements in said skirt conforming to the external threadformations.
 3. The invention as defined in claim 2, the depth of saidflutes being approximately the same as the depth of the external threadformations, whereby, subsequent to said constriction, the outsidediameter of said skirt across the major diameter of said thread elementswill be substantially the same as the diameter of said cylinder.
 4. Theinvention as defined in claim 3, the depth of said flutes being greaterthan the thickness of said material, whereby upon generally uniform saidconstriction of the entire said skirt below said first plane, the insidediameter of said skirt between said second plane and said edge will benot less than the major diameter of the external thread formations. 5.The invention as defined in claim 1, said skirt being weakened around acircle in a plane between said second plane and said edge.
 6. Theinvention as defined in claim 5, said cap being adapted for applicationto a bottle having a downwardly facing external shoulder below thethread formations on the finish, said skirt being of such length that alip portion thereof below said circle of weakening will extend below theshoulder when said cap is applied to the bottle, whereby said lipportion can be formed into engagement with the shoulder so as to form atamper-proof closure for the bottle.
 7. A twistably removable closurecap for engaging external thread formations on the finish of a bottle orthe like, said cap comprising: a top panel and a generally cylindricalskirt depending therefrom and terminating downwardly in an edge, atleast that portion of said skirt which extends from a first plane belowsaid panel to said edge being formed of sheet material of substantiallyuniform single thickness, said skirt being corrugated so as to definestraight outward ridges extending from said first plane to a secondplane spaced from said edge, there being flutes between said ridges,portions of said skirt along the lines of said flutes being conformed toelements of the external thread formations so as to define female threadelements between said ridges, the crests of said ridges and acircumferential band of said skirt immediately downward of said secondplane being elements of a hollow cylinder having an inside diameter notsubstantially less than the major diameter of the external threadformations, the other portions of said skirt below said first planelying substantially within the outside diameter of said cylinder.
 8. Theinvention as defined in claim 7, said ridge crests and the crests ofsaid female thread elements being rounded, the outside diameter of saidskirt across the major diameter of said female thread elements beingsubstantially equal to the outside diameter of said cylinder, saidridges, said thread elements and said band thereby defining a gridaround said skirt adapted to shield said edge and to afford a sure andcomfortable finger grip for twisting said cap off and on said finish.